1. Field of the Invention
The present invention relates to the field of semiconductor manufacturing devices and, more particularly, to chucks for controlling wafer temperature.
2. Prior Art
In the manufacture of semiconductor integrated circuit devices, various circuit elements are formed in or on a base substrate, such as a silicon substrate. Generally, the process of forming these various circuit elements starts from a base wafer, which is typically flat and is circular in shape. On each of these flat circular wafers, a number of integrated circuit devices, typically known as "chips" are formed by the use of various well-known techniques, including photolithography, doping, depositing, etching, and annealing techniques, just to name a few.
In performing some of these steps, a wafer is placed in a chamber in order for the wafer to undergo a necessary processing step, such as deposition or etching. When these wafers are loaded into a given chamber, the wafer is placed on a wafer chuck, which is a type of semiconductor platen. These platens, or chucks, are used to control the wafer temperature during a given process cycle. Because the wafer resides on the platen, by controlling the temperature of the platen, wafer temperature can be controlled. Accordingly, elaborate measures have been devised to address the various means available for controlling the temperature of the platen. Some of these prior art techniques are described in U.S. Pat. Nos. 3,501,356; 4,496,609; 3,669,812; 4,542,298; 4,628,991; 4,671,204; 4,457,359; 4,282,924; and 3,885,061. These patents teach a technique of cooling the wafer by circulating liquids, such as water. Either the cooling of the apparatus as a whole is provided by the circulating cooling water, or in more sophisticated systems, channels or passages are provided in the base of the chuck to directly cool the wafer chuck.
Another prior art device is described in U.S. Pat. No. 4,274,476, in which heat created inside the wafer is transferred to an expandable heat pipe, wherein the heat causes the fluid in the heat pipe to boil and vaporize. The vaporizable liquid is inside the cavity for expanding the heat pipe when heated and for transferring heat from one plate to the other. Furthermore, another scheme is described in U.S. Pat. No. 3,724,536, in which a fluid coolant, such as carbon dioxide, undergoes a rapid expansion upon entering an expansion chamber, thereby cooling the conductive element and consequently the device under control.
In practice, the temperature of the chuck must frequently be controlled at a temperature substantially below that of the wafer process temperature, especially when there is substantial energy input into the wafer. Substantial energy input to the wafer will usually occur when processing techniques such as plasma etch, chemical vapor depositions (CVD), and electron cyclotron resonance-chemical vapor deposition (ECR-CVD), just to name a few, are used. In some of these processes, the power input to the wafer can be as much as 8 W/cm.sup.2. As an example, for a 6-inch wafer, this is equivalent to a total power input of nearly 1500 watts. To maintain the wafer at the desired process temperature requires this amount of heat energy be removed from the wafer during the process. Thus, a substantial difference in temperature (.DELTA.T) between the wafer and the chuck must be maintained in order to realize the required heat energy extraction from the wafer.
In a prior art system utilizing the circulation of cooling water, a considerable amount of cooling water flow must be maintained in order to dissipate the heat generated by such energy input. For example, an 1800 W energy input system would require approximately 0.5 liter/second (6.6 gallons/minute) flow of cooling water having a .DELTA.T value of 1.degree. C. from inlet to outlet. Thus, considerable amount of cooling water must be circulated in order to dissipate the required energy. Although it is possible to substitute other cooling fluids to reduce the required volume of flow of the coolant, significant amount of liquid is still needed and the liquid must be maintained in a closed system for recirculation. In many instances condensation or other processes for reclaiming the liquid is needed within the closed system.
Furthermore, with most prior art closed loop systems, fluid passages are typically present within the wafer chuck to circulate the cooling fluid in order to dissipate the heat from the chuck. Additionally, in a closed loop system, the temperature of the circulating fluid will typically need to be controlled. In some instances where an expansion chamber is used, such as in a system utilizing liquid gas which is expanded to remove the heat, a sophisticated closed loop system must be present in order to control the temperature of the chuck, as well as maintaining the proper flow of a specially designated coolant, other than water, to the wafer chuck.
In those special processes, such as plasma etch and ECR-CVD processes, additional temperature control problems are encountered. These processes deposit substantial amounts of energy into the wafer and the wafer must be cooled by the wafer chuck to keep it at the selected process temperature. For example, in one CVD-SiO.sub.2 deposition process, the temperature of the chuck is controlled to a value in the range of 65.degree. to 90.degree. C. by circulating thermostated liquid through the base of the chuck. If a cold (room temperature) wafer is loaded onto the chuck, the wafer will be heated to the temperature of the chuck within a few seconds. The wafer temperature, however, is still 200.degree. to 400.degree. C. below the optimum deposition temperature. If the film deposition is begun before the wafer is at this temperature, the quality of the initial layer of the film will be inferior to that deposited at the optimum process temperature. Alternatively, with no silane flow into the reactor the plasma can be used to heat the wafer without deposition, but at the cost of an additional 15 to 20 seconds added to the process. For a 120 second process that represents an increase in process time of about 16% or a proportional decrease in yield in the number of wafers that can be processed per hour.
For maximum throughput of the tool in certain high energy processes, such as plasma processes, it is imperative that the wafer be brought up to its operating temperature as quickly as possible and once the operating temperature has been reached, to remove the process generated heat from the wafer in a controlled manner. In order to provide these objectives, it is preferred that a chuck be designed to accommodate the thermal shock of drastic temperature changes of the order of 200.degree. C./sec and yet maintain the ability to dissipate heat energy in order to control the temperature of the wafer. In such a design, it is preferable to design an economical, yet efficient system, to reach the desired objectives.